Downholes (underground drilling pits) are prepared for recovery of hydrocarbon resources including oil and gas (representatively called “oil” sometimes hereafter) from the underground. Downhole tools such as frac plugs (disintegratable plugs), bridge plugs, cement retainers, perforation guns, ball sealers, sealing plugs, and packers (inclusively referred to as “downhole tools” hereafter), are used for the formation or repair of the downholes. Thereafter, the tools are often disintegrated or allowed to fall down without recovery onto the ground. (Examples of such downhole tools and manners of use thereof are illustrated in, e.g., Patent documents 1-5). Therefore, it has been recommended to form the whole or a component thereof constituting a bonding part allowing collapse (i.e. downhole tool member) with a degradable polymer for the tool of such temporary use. Examples of the degradable polymer may include: polysaccharide, such as starch or dextrin; animal albumin polymers, such as chitin and chitosan; aliphatic polyesters, such as polylactic acid (PLA, typically poly L-lactic acid (PLLA)), polyglycolic acid (PGA), polybutyric acid, and polyvaleric acid; and further, polyamino acids, polyethylene oxide, etc. (Patent documents 1 and 2). However, the technology of designing the change of mechanical strength under degradation and time to collapse of the downhole tool member by using the degradable polymer has not been satisfactorily developed because it was difficult to accurately evaluate the degradation behavior of the degradable polymer.